1. Field of the Invention
The invention relates to non-conductive substrates for supporting printed circuits, methods of manufacturing such substrates and printed circuit units made therefrom.
2. Brief Description of the Prior Art
Prior to the present invention, numerous methods were available for the manufacture of printed circuit units. One widely used method is the so-called "subtractive method" wherein printed circuit substrates (boards) are made with circuit holes in or through the board. The board is a non-conductive base and may bear a conductive circuit on one or more surfaces of the non-conductive base, connected through the circuit hole. The method of making such units comprises adhering an electrically conductive metal foil to the non-conductive base, applying a photo-resist coating to the foil, curing the coating in a predetermined pattern, washing away the uncured coating and etching the exposed portion of the metal foil away. Left are the portions of foil in a desired circuit pattern corresponding to the predetermined pattern. The walls of the circuit hole joining patterns on different surfaces receive a deposit of palladium crystals, which serve as a nucleating site on which a conductive connection is made from one surface to the other by a copper plating. This thin copper plating must be strengthened galvanically with very ductile copper coatings. In this manner, the copper is deposited on the circuit hole walls and also on the conductor circuit. As mentioned above, in the etching process the conductor paths or circuit is covered with an etch resistant material. During etching, the uncovered conductor faces are etched off with etch solvents. The disadvantage of this so-called "substractive method" is that it is time consuming due to the numerous operating steps. Furthermore, a large quantity of copper is accumulated in the etch solvents, and cannot be used further or must be subjected to a rather expensive recovery process. Furthermore, insufficient etching may occur.
In the so-called "additive method" an adhesive is applied to the non-conductive circuit board and then subjected to a chemical etching treatment so as to provide bonding between a subsequently applied copper layer and the non-conductive board. Subsequently, the non-conductive board is sensitized and then copper plated in a known manner. Thereafter, also in a known manner, a negative pattern is made on the copper plate, employing a screen printing or photo printing process. In a subsequent galvanic copper plating the desired conductor path thickness may be obtained. The subsequent operating steps correspond to the ones in the above-described subtractive method. For obtaining a better bonding of the conductive circuit to the base, it is often necessary to first roughen the non-conductive board's surface prior to its receiving the adhesive. It will be appreciated that the "additive method" has many of the same disadvantages associated with the "subtractive method".
It is within the state of the art to apply metal patterns or conductor paths on a non-conductive substrate by means of an electroless metal deposition. For this purpose the non-conductive substrate surface must be sensitized or activated. A general method for activation comprises bringing the substrate into contact with two separate solutions. First, the substrate is brought into contact with an aqueous acid solution of a reducing agent followed by a rinsing with water. The substrate is then treated with a second solution of an active metal salt in dilute hydrochloric acid. The reducing agent, which is absorbed by the substrate surface in the first step, reduces the ions of the active metal on the substrate surface, whereby sensitized centers are generated on which the chemically deposited metal adheres. This method is disadvantageous in that the metal deposits, for example copper coatings do not bond sufficiently to the non-conductive substrate surface. A further disadvantage is that, in a two stage activation of the substrate, after the activation and before carrying out further treatment steps, the non-conductive substrate must be transferred to other supports so as to eliminate a contamination of the activator solution and a rapid decay of the metal deposition bath. In addition, it is very hard to cover the circuit holes with a deposit of metal.
A method is also known for making a printed circuit board wherein electrically conductive faces and conductor paths are applied to a non-conductive substrate layer by electroless metallization. In this case the electrically non-conductive substrate may be a phenolic resin or an epoxide resin mass which contains an inorganic filler material in addition to a copper oxide. The filler material may be treated with acids or lye. The filler materials may be metal oxides and/or silicon dioxide (which can be obtained by wet precipitation as well as in a pyrogenic process). Furthermore, synthetic alkali and/or earth alkali and/or aluminum silicate may be used. The filler material is added to the electrically non-conductive base material in quantities of preferably 20 to 40% by weight. After making pressed plates of the filled, non-conductive materials, the surface of the plates are degreased in a conventional alkali degreasing bath, and subsequently the pressed plate is treated with a warm sodium hydroxide solution. A treatment with sulfuric acid is then carried out so as to partly transform the copper oxide on the surface of the pressed plate into copper. A thin copper plating on the plate is then carried out in a plating bath. The desired faces on the prepared plates are then galvanically strengthened in that the remainder of the faces are first covered with a protective lacquer. The protective lacquer is then removed in a desired pattern and the thin, nonstrengthened copper layer is etched off. While the mixture of phenolic resin mass and copper oxide has a low metal bonding characteristic, the adherence values for the mixture of a phenolic resin press mass, copper oxide and a filler material are higher. However, this method can only be used for an electroless metallization on the surface of the plate, but not on the inner walls of the circuit holes. Furthermore, the use of thermoplastic materials as the non-conductive base is generally disadvantageous since, although conductor paths may be mounted thereon, no resistor layers (which have to be hardened at higher temperatures) can be affixed. Furthermore, the processing of thermoplastic materials in a press molding process is relatively cumbersome and therefore expensive.
A method for making a plastic, electrically nonconductive element is known onto which a metal layer may be mounted. The plastic element comprises an epoxy resin or a polyester resin. While in a uncured state, a finely dispersed catalyst salt is added to the resin. The catalyst salts may be the salts of metals of group VIII or group XII of the Periodic Table. The catalyst salt, which is generally found to be in higher concentration on the surface of the plastic element rather than in the inner layers, is activated during the curing or hardening of the plastic material. Filler material, in powdered form, consisting of tartaric acid, citric acid or acetic acid are also admixed in the uncured plastic material. The surface of the plastic material is then removed after a partial hardening, mechanically or chemically, until the powder granules of metal salts are partially free and are removable with a solvent. The metal salt powder granules, which are reduced to metal subsequently, initiate a further chemical metal reduction during the autocatalytic metal deposition. The disadvantages of this method is that a continuous, non-controllable removal of the metal salts takes place in the metallization bath. Obviously, this method is suitable for metallization of a plastic surface but not for the walls of any circuit holes passing from one surface to another surface in the non-conductive base.
The present invention provides a non-conductive board for supporting a printed circuit and a method of its manufacture which removes many of the aforementioned problems and disadvantages of the prior art. The circuit board of the invention comprises an easily processed support material with conductor paths having an increased adherence to the support. The printed circuit board of the invention is also advantageous for mounting resistor layers.